1. Field of the Invention
The present invention relates to an ignition timing control system for internal combustion engines such as gasoline engines, or more in particular to an ignition timing control system for internal combustion engines having a function to prevent over-revolutions of the engines.
2. Description of the Prior Art
A phenomena known as over-revolutions of an engine, especially, those of a reciprocal engine, is a cause of such trouble as valve surge adversely affecting the engine durability. To prevent this trouble, conventional ignition systems or fuel supply systems are equipped with an over-revolutions prevention mechanism (such as disclosed in JP-A-61-164077) for retarding the ignition timing or stopping the ignition. Other conventional systems for internal combustion engines using an electronically-controlled fuel injection unit comprise means for stopping fuel injection to prevent over-revolutions. In internal combustion engines using a carburetor, on the other hand, it is very difficult to stop the fuel supply completely and therefore the very ignition is required to be stopped.
These conventional systems with an over-revolutions prevention mechanism, which operate to search for an ignition retardation angle set in advance in accordance with the engine speed in over-revolutions range, however, have the disadvantage that the over-revolutions prevention mechanism cannot be optimally operated in a versatile manner in accordance with variations in environmental conditions, secular variations or engine variations.
Another disadvantage of these conventional systems is that if ignition is stopped to prevent over-revolutions of an internal combustion engine having a carburetor using distributorless ignition (DLI) of simultaneous ignition coils (W coils), an uncombusted gas would be discharged with the engine speed reduced, with the result that the waste fire due to the simultaneous ignition would be caused at the end of exhaustion stroke after restoration of ignition, thereby igniting the uncombusted gas in the exhaustion stroke. When an intake value is opened, therefore, the uncombusted gas in the intake manifold is also ignited to cause a backfire, which is a frequent cause of a damaged intake system due to combustion and an increased pressure in the engine intake system.